Distance between metal-binding sites in transferrin: energy transfer from bound terbium(III) to iron(III) or manganese(III)

Abstract

The distance between the 2 metal-binding sites of human serum transferrin was studied by observing energy transfer between an excited Tb ion bound at one site and a ferric (or manganic) ion bound at the other site of the same transferrin molecule. From the observed reduction in Tb lifetime (relative to that of Tb transferrin), it is concluded that the intersite distance is 3.55 .+-. 0.45 nm. This distance is reconciled with 2 conflicting earlier reports that the separation between sites is greater than 4.3 nm [Luk, (1971)] or is equal to 2.5 .+-. 0.2 nm [Meares, and Ledbetter, (1977)]. The difficulty of accurately measuring the quantum yield of protein-bound Tb provides the principal source of uncertainty in these measurements.